1. Field of the Invention
The present invention relates to a backlighting lamp and a method of lighting and, more particularly, to a backlighting lamp and a method of backlighting for use in liquid crystal display panels.
2. Description of the prior art
As a light source for a backlighting device in liquid crystal display panels used in such devices as a LCD television, it is conventional to use an electroluminescence (EL) or a fluorescent lamp. An advantage of the electroluminescence is that a luminous body can be made flat and thin and its light distribution made uniform. However, the electroluminescent involves technical problems such as a lack of luminous intensity or inability to produce a desired white light and is generally regarded as unsuitable for use in the backlighting for liquid crystal display panels. Thus, the backlighting device which utilizes as its light source a fluorescent lamp of a straight pipe type or a U-shaped pipe type and in which luminous brightness is higher than that in the electroluminescent, has been used in many practical applications.
Examples of such conventional backlighting devices are shown in FIGS. 1 and 2. The numeral 1 represents a housing 1 whose inner surface is a reflective surface, 2 represents a straight pipe type fluorescent lamp which extends through the housing 1, and 3 represents a diffuse plate which closes an upper opening of the housing 1. When the fluorescent lamp 2 is lit, the light is diffused and transmitted through the diffuse plate 3 and the liquid crystal display panel 4 on the diffuse plate 3 is illuminated. Since the reflective surface of the housing 1 is curved and the diffuse plate is present, the panel 4 is illuminated with fairly sufficient uniform light distribution.
As another backlighting device, there has been proposed a flat type electroluminescent lamp as shown in FIGS. 3 and 4. This is constituted by a fluorescent film 6 formed on an inner surface of a rectangular plate type housing 5, a pair of electrodes 7, 7 provided at opposite two inside ends of the housing 5, and a transparent glass plate 8 airtightly closing the upper opening of the housing 5. The electrodes 7, 7 are led out by external leads 9, 9 from both ends of the housing 5. Inert gas and mercury-vapor are enclosed in a sealed space formed by the glass plate 8 and the housing 5. The pair of electrodes 7, 7 are of either a cold cathode type or a thermal cathode type and extend transversely of the housing 5. When a voltage is applied to the pair of electrodes 7, 7, a flat glow discharging develops extending transversely of and within the housing 5 thereby causing the fluorescent film 6 to glow. The liquid crystal display panel 4 on the glass plate 8 is illuminated with the light being radiated externally from the glass plate 8 with fairly sufficient light distribution.
Next, a conventional method of backlighting is explained. As shown in FIG. 5, in the backlighting device, a straight pipe type fluorescent lamp 2 extends through the housing 1 whose inner surface is a reflective surface and the upper opening of the housing 1 is airtightly closed by the diffuse plate 3. The fluorescent lamp 2 is lit when a high frequency voltage is applied to its electrodes from the power source 10.
The backlighting device shown in FIGS. 6 and 7 is of the type in which a plurality of straight pipe type fluorescent lamps 2, 2 are used. In this arrangement, a plurality of fluorescent lamps 2, 2 disposed in parallel are connected to one power source 10 and a stabilizer 11 is provided to each of the fluorescent lamps 2, 2.
The backlighting device shown in FIG. 8 is constituted by a fluorescent film formed on the inner surface of the housing 12, a pair of electrodes 12 disposed at opposite inside ends of the housing 12, and a transparent glass plate 14 airtightly closing the upper opening of the housing 12. Inert gas and mercury are filled in the sealed space formed by the housing 12 and the glass plate 14. In this backlighting device, when a high frequency voltage is applied from the power source 10 to the pair electrodes 13, 13 at both the ends of the housing 12, a flat glow discharge develops extending transversely or widthwise of the housing 12 so that the fluorescent film glows. This resultant light is radiated externally from the glass plate 14 and the liquid crystal display panel on the glass 14 is illuminated.
In the backlighting device described above and shown in FIGS. 1 and 2, since the housing 1 is provided with a curved reflective surface and the diffuse plate 3 is required, the overall structure inevitably becomes thick and heavy thereby making it difficult to meet the demand for a lighter and more compact backlighting device adapted to be used in such as a portable television having a liquid crystal display. Moreover, the curving of the reflective surface, through which it is possible to obtain comparatively uniform light distribution, involves the difficult technology which presents limitations in the designing of backlighting devices.
On the other hand, in the backlighting device shown in FIGS. 3 and 4, although it does not require either the reflective plate or the diffuse plate, the sealed space formed by the housing 5 and the glass plate 8 contains the inert gas and mercury almost or substantially in a vacuum state, so that the glass plate 8 and the housing 5 which are exposed to atmospheric pressure must inevitably be thick enough resulting in the overall structure becoming heavy and presenting difficulties in meeting the need for making the backlighting device light and compact. It may be attempted to provide some supporting members at appropriate points within the sealed space between the housing 5 and the glass plate 8 in which case it may be possible to make the glass place 8 and the housing 8 thinner to some extent. However, the problem in such attempt is that, because of the presence of the supporting members, the luminosity deteriorates and the light cannot be distributed uniformly.
In the backlighting devices shown in FIGS. 6 and 7, one advantage is that, because there are a plurality of fluorescent lamps, it is possible to obtain the light distribution more uniformly than in the backlighting device shown in FIG. 5. However, a problem therein is that, where a single light source 10 is to be used for lighting up the plurality of fluorescent lamps, each of such lamps must have an independent stabilizer, which adds to manufacturing costs and results in the need for an increase in the size and weight of the device. A further problem is that the individual fluorescent lamps each have different electric characteristics because of variations such as in the length of the discharge channel, the diameter thereof and the pressure of the gas contained therein. The unbalanced characteristics in the individual fluorescent lamps result in differences or non-uniformity of lighting and in difficulties in obtaining the necessary uniform light distribution.
Also, in the backlighting device having a fluorescent lamp shaped in "U" or "W" in its cross section, there is an advantage for the device to have more uniform light distribution and to be more compact and lighter than in each of the above described backlighting devices. However, since the device with the "U" or "W" shaped lamp has a discharge channel in a serpentine form, the distance between two electrodes is long and this is especially required to be larger for more uniform light distribution and large display areas. The larger distance requires a higher discharge starting voltage which results in a shorter lifetime of the device and in a higher cost for the provision of the necessary drive circuitry.